Antibodies to the class 2/3 OMPs (PorB) are bactericidal and antigenic diversity between these proteins forms the basis of the current stereotyping classification. The variable regions of the porB gene from the 13 serotype strains expressing class 3 OMP have been examined and a panel of oligonucleotide probes for identification and classification of strains using genetic techniques developed. We have developed a PCR system to examine the molecular epidemiology of meningococcal meningitis in Brazil by amplifying porB directly from cerebrospinal fluid with subsequent oligonucleotide probe hybridization to genotype the porB variable regions. We are applying this system to CSF samples from 100 patients with meningitis from Brazil. Using both probe hybridization and direct sequencing, we have identified regions of the porB gene from an unusual Chilean strain (501) which are similar to each of the two prototype type 4 and type 15 strains. Rabbit immunization studies using this strain are suggesting immunodominance of epitopes corresponding to particular variable regions in individual animals. Anti-class 3 OMP (PorB) antibodies from animals immunized with strain 501 bind only to serotype 4 and strain 501 class 3 OPM but not to serotype 15 OMP and are therefore directed against the epitope corresponding to VR1. However, antibodies from serotype 15 OMP immunized animals binds equally well to type 15 and strain 501 class 3 OMP suggesting that at least a substantial portion of these antibodies are to epitopes coded for by VR2, VR3, or VR4. Bactericidal assays coincide with the data from Western blots, and using a type 4 strain which does not express class 1 OMP (PorA), the bactericidal nature of antibodies to the epitope coded by VR1-4 has been shown. We are currently typing strains from New Zealand using these class 3 probes, and have determined the variable region types of some strains which were non-serotypeable. These studies will help in our understanding of how host immune pressure causes selective changes in meningococcal surface proteins, and will help in out understanding of the nature of non-capsular protective immunity to meningococcal disease. These studies have resulted in development of a genotyping system that may be used to evaluate cases of meningococcal disease in vaccine trials, including culture negative suspected cases.